The research team led by Zheng Hao and Jia Jinfeng of the School of Physics and Astronomy of Shanghai Jiao Tong University successfully generated and detected the segmented Fermi surface caused by Cooper's momentum in the Bi2Te3/NbSe2 system using a low-temperature strong magnetic field scanning tunneling microscope, and the paper was accepted by Science and selected as First Release to be published online in the early morning of October 29, 2021.
This work innovatively uses the particularity of topological insulator/superconductor heterojunction to solve the difficulties in the experiment, for the first time observed the segmented Fermi surface predicted by the theory more than 50 years ago, and found that the shape and size of this Fermi surface can be adjusted with the direction and size of the magnetic field, and can also regulate the topology, build a new topological superconductivity, and open up a new way to regulate the state of matter.
The basic knowledge of solid state physics tells us that the density of states near the Fermi surface of materials determines whether they conduct electricity, whether they are transparent and other physical properties. The traditional state regulation of matter is to regulate the density of the state near the Fermi surface, if the manual regulation of the Fermi surface can be realized, it will bring revolutionary changes to the regulation of the physical properties of the material.
Superconductors have strange properties such as zero resistance conductivity and complete antimagnetism, and are a long-standing research topic in physics; due to the existence of superconducting energy gaps at the Fermi energy level, superconductors have no Fermi surface. As early as 1965, Fulde theoretically predicted that if the momentum of the Cooper pair was large enough, quasiparticles could be produced in the superconducting energy gap, resulting in a special kind of "segmented Fermi surface" [Phys. Rev. 137, A783-A787 (1965)]。 However, since the ordinary superconductor Cooper is large enough for momentum, at the same time as the quasiparticles are generated, the Cooper pair will also rupture and lose superconductivity, so it is very difficult to observe this "segmented Fermi surface" experimentally. Although more than 50 years have passed, this prediction has not been confirmed by experiments.
The research team used molecular beam epitaxial technology to accurately grow a 4-layer thick topological insulator Bi2Te3 film on the surface of the superconductor NbSe2. In this system, due to the large Fermi velocity of the Surface State of Bi2Te3, the Momentum of Cooper pairs in the Surface State of Bi2Te3 is already very large when the Momentum of Cooper in the NbSe2 superconductor is still very small (see Figure 1A). In this way, a small horizontal magnetic field can be used to generate a small superconducting current on the surface of NbSe2, but at this time the Cooper pair momentum in the Surface State of Bi2Te3 is enough to produce quasiparticles, and leads to the emergence of segmented Fermi surfaces, which cleverly solves the experimental difficulties. They used a scanning tunneling microscope equipped with a dilution chiller and a three-dimensional vector strong magnetic field to conduct the study, as shown in Figure 1, as the magnetic field increases, the Momentum of the Cooper pair increases, and more and more quasiparticles in the superconducting energy gap (the bottom of the energy gap is becoming less and less zero, see Figures 1D and 1E), indicating the gradual emergence of segmented Fermi surfaces in the superconductor.
Figure 1. A. Bi2Te3/NbSe2 superconducting heterojunction diagram. B. High-quality Bi2Te3 film topography grown on NbSe2 substrate. C. Atomic resolution diagram of a Bi2Te3 film. D, E. Under the action of intraplanar magnetic fields of different sizes and directions, the signals derived from quasiparticles in the tunnel spectrum gradually increase.
Further, using quasiparticle interference (QPI) techniques, the team detected standing waves in real space (see Figures 2A-C, G-I), and confirmed the generation of Fermi planes at zero energy through the Fourier transform. It is worth noting that the fermi surface is composed of a part of the non-superconducting Bi2Te3 fermi surface, and its shape and orientation can be determined by the strength and direction of the applied magnetic field (see Figure 2D-F, J-L), which is fully in line with the characteristics of the segmented Fermi surface of the superconductor predicted by theory.
Fig. 2, A to C and G to I, the standing wave pattern generated by real space after applying a magnetic field of 40 mT in the direction of Γ-K and Γ-M, respectively. D to F and J to L, corresponding Fourier transforms are performed on A to C and G to I, producing quasiparticle interference signals dependent on the direction of the magnetic field.
Author: Ye Dan Gao Lu
Editor: Chu Shuting